Oxygen permeability is one of the main weaknesses for the clinical application of biological hydrogel membranes, e.g., immunoisolatory membranes, extended-wear soft contact lenses. Investigations on novel fully synthetic membranes for biomedical applications, including extended wear soft contact lenses, necessitate the determination of the oxygen permeabilities of water swollen membranes rapidly, precisely and reproducibly. Pertinent literature reveals that the required methodology for the determination of high (Dk>100 barrers) oxygen permeability water-swollen hydrogels is unavailable or fraught with other issues.
One method for determining the oxygen permeability of water-swollen hydrogels is the Fatt method (International Standard ISO 9913-1: 1996(E)). However, this analytical technique is only reliable in the Dk equal to the 1 to 100 barrer range. Given the above this technique is inadequate to provide accurate values for highly oxygen permeable silicon-based contact lenses, such as lotrafilcon (Dk=140 barrers) and balafilcon (Dk=110 barrers). The Fatt method is unsuitable for the determination of high oxygen permeabilities because of insufficient understanding of the electrode/polymer (solid/solid) interface and unpredictable edge effects at higher permeabilities.
Since the introduction of modern silicon-based hydrogels for extended-wear soft contact lenses in 1997 several attempts have been made to develop techniques for the measurement of oxygen permeabilities above the 100 barrer value. A methodology has been developed in which the oxygen permeability of water-swollen soft contact lenses placed between two diffusion chambers is determined by the use of a chromatographic oxygen sensor. The detector used for the determination of oxygen concentration is part of a HPLC (High Performance Liquid Chromatography) system and samples are taken from the diffusion chambers and injected into the HPLC. One advantage of this methodology is that the analyzing unit is isolated from the diffusion chambers and hence is independent from the measurement conditions. The oxygen sensor is capable of detecting oxygen in the range of 0.01 mg/L to 10.0 mg/L. The reproducibility and the reported error for both low and high oxygen permeability contact lenses were quite low (6 to 10%). However, the Dk values for the high permeability lenses had a 20 to 30% error margin when compared to the values claimed by the manufacturers and those determined by other suitable testing methods. Although the above method claims steady state conditions are not needed for accurate measurements, the equations used in this method presume such conditions. Also, this method utilizes a water phase HPLC system, and such an HPLC system and a detector having the necessary sensitivity are expensive, thereby limiting the use of this technique.
Alvord et al. (Optometry & Vision Science, 1998, 75(1), pp. 30 to 36) determined the oxygen permeability (Dk) and transmissibility (Dk/l) of lotrafilcon A lenses by a modified standard coulometric method. The coulometric method is the ISO standard for measuring the oxygen permeability of rigid contact lenses but cannot be used for soft contact lenses. Contact lenses with different thicknesses were measured in liquid-to-gas and gas-to-gas configurations in an effort to combine the features of the Fatt method with the advantages of the coulometric method. Oxygen permeabilities were measured with the same or less error than the Fatt method is capable of and the results were within 10% of the nominal values with low relative errors. Although this method is theoretically suitable for the determination of oxygen permeabilities with Dk values of 200 or more barrers, there is no concrete evidence that such measurements are in fact accurate.
If it were possible to determine oxygen permeability values above a Dk of 200 or more barrers, another possible problem with the Alvord test is that the Alvord test may yield data that is unreliable due to a failure to properly account for the edge effect and boundary layer effect.
PDMS (polydimethylsiloxane) occupies a special position among highly oxygen permeable materials. It has by far the highest oxygen permeability (Dk value) among rubbers and has the second highest Dk among all polymers (currently the highest Dk of all polymeric materials is exhibited by poly[1-(trimethylsilyl)-1-propyne]). The oxygen permeability of PDMS is typically determined in the dry state using a well-known method for the testing of gas separation membranes. According to various literature sources the oxygen permeability of unfilled dry PDMS is in the Dk equal to the 700 to 900 barrer range. The relatively wide range of reported Dk values is due to differences in sample preparation, differences in the relative crosslink densities of the test pieces, extrapolation from filled PDMS to unfilled PDMS, presence of impurities, etc, but not to measurement conditions. In spite of the Dk equal to the 700 to 900 barrer range reported for PDMS, most contact lens literature gives Dk's in a significantly lower 200 to 600 barrer range.
There is a need in the art for a technique by which to determine the oxygen permeable values for highly oxygen permeable water-swollen hydrogels (e.g., PDMS containing polymer membranes).